Recently, several refractometers based on the transmission of light through a sensor have been produced for measuring different physical properties of a medium, such as, for example, the index of refraction of a fluid. Most of such refractometers use an optical fiber in which the sensing element is a curved section of the fiber. An optical fiber generally has a core of transparent material of refractive index nco surrounded by a layer of transparent cladding material having a refractive index n2 which is lower than nco. The core of the curved section is exposed to the medium to be analysed. Such a refractometer is generally provided with a light source that illuminates the sensing element, a photodetector for detecting the light transmitted via the sensing element and a processor with which the refractive index of the fluid can be determined from the light that reaches the photodetector. Most of these prior art devices describe a sensing element which is U-shaped.
For example, A. L. Harmer of Battelle Memorial Institute obtained a series of patents in the early 1980's (U.S. Pat. Nos. 4,187,025; 4,240,747; 4,427,293; and 4,433,913) which disclose several fibre optic sensors in which the sensing element is a curved section of the fibre. More particularly, U.S. Pat. No. 4,433,913 describes an apparatus for determining the index of refraction of a fluid including a light source, a light detector and means forming a bent light-conducting path between the source and the detector such that the intensity of the detected illumination is representative of the refractive index of the fluid surrounding the light-conductive path. Harmer also mentions that a bent light-conducting path having more than one curved section will provide a higher sensitivity than a single curved section.
Also of interest is U.S. Pat. No. 4,468,567 issued to N. Sasano et al. disclosing a liquid level detecting device having a low transmission loss. This liquid level detector includes two or more optical fibres which are placed such that their ends are adjacent either in a parallel relationship or forming an acute angle with each other. The sensing element of Sasano consists of a spherical portion formed by fusing the end portions of the optical fibres.
R. Omet in U.S. Pat. No. 4,564,292 discloses a refractometer which uses two U-shaped optical fibres, one serving as the reference and the other for the sample to be analysed. Schoch et al, in U.S. Pat. No. 4,639,594, uses the same two fibres configuration for a liquid level sensor, wherein a reference liquid is sealed into the reference chamber.
In U.S. Pat. No. 4,806,013, J. S. Bodenheimer et al. describe the use of a single U-shaped optical fibre as a refractive index sensor wherein the bent portion must have a radius of curvature at least ten and at most two hundred times larger than the fibre diameter. They state that the low sensitivity of the U-sensor mentioned by Harmer and others is due to the selection of a too small bent radius. They give an empirical formula for calculating the minimum bent radius for various indices of refraction.
In U.S. Pat. No. 4,851,817, C. E. Brossia et al. disclose a system for automatic and real time detection of water and icing on surfaces by monitoring variations in light energy transmitted through an optical fiber having a sensitive probe area being U-shaped. The sensitive probe area is positioned on, about or within the surface on which icing is to be detected. Because of differences in optical indices of refraction and energy absorption characteristics of air, water and ice, the presence of each of these at the processed sensitive area will cause a proportional and characteristic attenuation of the light energy passing through the optical fiber. Changes in light energy transmission can be interpreted automatically to provide an indication of icing.
In U.S. Pat. No. 5,141,310, A. A. Boiarski discloses a sensor for measuring the specific gravity of a liquid, in particular urine, wherein the sensing element, which can be a specifically shaped optical fiber, is provided with multiple reverse bends. The sensing element measures the refractive index which is then related to specific gravity.
W. A. Stevenson et al., in U.S. Pat. No. 5,585,634, disclose the use of an optical fiber as a multiple internal reflection (MIR) sensor for performing both emission spectroscopy and absorption spectroscopic measurements of highly absorbing or highly scattering material, a technique sometimes referred to as attenuated total reflectance (ATR) or evanescent wave spectroscopy. More particularly, this patent discloses a U-shaped fibre optic sensor for spectroscopic monitoring wherein the core at the U is exposed to the material.
U.S. Pat. No. 5,699,461 issued to H. Minemoto et al. discloses the use of a U-shaped optical fiber which fits into the groove pattern of a substrate. The applications of such a sensor are magnetic field/current, electric field/voltage and temperature sensors.
G. Borak et al., in U.S. Pat. No. 6,003,340, describe a method by which a fiber is bent into a multiple bends shape by heating with a laser beam and by which method they produce a sensor sensitive to strain.
Even if all these prior art devices seems to well fulfil their task, the attenuation of light due to the bending of the optical fiber complicates the analysis of the light reaching the detector.
Other types of refractometers have also been developed. An example is U.S. Pat. No. 4,994,682 issued to S. H. Woodside which discloses a liquid level sensor using two optical fibres to detect liquid levels. There are notches along the length on one side of each of two fibres. The fibres act independently and thus serve for ratio measurements. In one embodiment a V-shaped substrate acts as a waveguide between the two fibres.
Another patent that mentions the use of a notched optical fibre for the measurement of refractive index is U.S. Pat. No. 6,130,439 issued to M. LeMenn. In this patent, M. LeMenn describes a sensor element which includes a machined groove and a surface which is parallel to the groove. The sensing principle is the detection of interference fringes, the movement of which is a function of refractive index changes of the liquid surrounding the sensor.
All of the devices discussed above present the major drawback that the light reflected by the sensing area is quite complicated to analyse due to the multiple reflections of the light beam at the level of the interface. Therefore, there is a need for a more simple sensing device that would overcome the disadvantages of the prior art devices by providing a sensor wherein a single reflection occurs at the level of the sensing area.
On the other hand, optical waveguides, which are well known transmission medium for optical communications systems also are generally optical fiber based. According to their construction, optical waveguides cannot accept important curvatures without generating significant attenuation losses. Indeed, bending an optical fiber beyond a particular curvature will generate some guiding losses which will increase with the curvature of the optical waveguide. The more the optical fiber is bent, the more guiding losses increase. Although the theory of optical waveguides has been known for a long time, practical optical waveguide devices that can be bent without generating important transmission losses are still unavailable. More particularly, the increasing need for miniaturised optical communication systems using optical waveguides requires more efficient bent waveguides in order to achieve an improved miniaturisation. A efficient bent waveguide may also be useful in many other applications.
There is therefore a need for an integrated bent optical waveguide device that would be able to transmit a light beam from one direction to another without the transmission loss that occurs with the conventional optical couplers actually available.